benq FP71Gplus monitor repair cant find problem

[rauno45]

Hello,
so i had this monitor that would not turn on. I looked inside and found some bad caps and replaced them. now the monitor turns on, but only shows the picture after the benq logo for a second and then powers off. When i power it on again i only see benq logo, no picture appears and then it powers off. when i power it on again, then benq logo is again displayed but for the shorter amount of time. I suspect the error to be somewhere on the power supply board. I think it could not be the inverter, because the monitor does not eaven turn on properly. apart from the electrolytes i replaced i have also checked some of the other capacitors just in case. They seem to be fine, and i found nothing uncommon there. I have attached a picture of the power board. Any help would be apreciated, thanks.

[UniT3d]

Re: benq FP71Gplus monitor repair cant find problem

Hmmm .....

1. Read https://www.badcaps.net/forum/showthread.php?t=12701 for pointers ...

2. Recommend to replace all secondary caps now rather than later

3. Check picofuse PF801

4. Check Q808, Q809, Q815, Q816 for shorts (2sc5707). Must replace in pairs if shorted.

5. Check Q805, Q812 for shorts (FU9024)

6. Check (& resolder) C824, C826

7. Clean and resolder T801, T802, T803, T804

[rauno45]

Re: benq FP71Gplus monitor repair cant find problem

AAh, yes, i found that the transistors that are around c826 have shorted legs plus c826 and c824 are possibly blown. q805 and q812 seem to be fine and pf801 seems to be fine. i think will replace c826,c824 and q808,q809 and see what happens.

[rauno45]

Re: benq FP71Gplus monitor repair cant find problem

I ordered the 2sc5707 and the caps. ill reply here when i have replaced them.

[rauno45]

Re: benq FP71Gplus monitor repair cant find problem

Well, it seems like that is not the problem. i replaced the 2sc5707 transistors but later measurments showed that they were shorted again, so that means they are shosted somewhere and the transistor itself is ok. also the fu9024 testing turned out fine, c824 and c826 are also fine, i have resoldered the inductor legs, picofuse is ok and secondary caps also seem to be ok. I think if i would be able to find the shorted part that shorts the 2 2sc5707 transistors then i will be good to go. (q808 and q809 are shorted somewhere on the scheme). I also found that the power connector had cold solder joints so i fixed them too.

[selldoor]

Re: benq FP71Gplus monitor repair cant find problem

Check the 5707s off the board - did you test them before you put them in?
Are they the right way round, - Pictures are not really clear enough for us to help troubleshoot.

[rauno45]

Re: benq FP71Gplus monitor repair cant find problem

well yes i tested the 5707 before i put them in, they worked ok(tested in diod mode) and did not beep in continuity mode, however if i solder them in the board and then measure continuity, the tester beeps showing short circuit. i soldered them out again and measuring the c5707 then again worked fine.Then unfortunate for me, when i soldered it back in, my cheap a## soldering iron made a mess burning some contact points off the board so i had to improvise a bit, but that did not change anything and i am back where i started. ill uploaded some pics from my shitty camera/mobile which were the best i got.

[rauno45]

Re: benq FP71Gplus monitor repair cant find problem

Yey, i got it working tonight. the mistake was where i had melted off the contacts. bypassing them made it all good. Therefore a conclusion: never do anything with a cheap a## soldering iron.

[selldoor]

Re: benq FP71Gplus monitor repair cant find problem

Well done! cant just make out your "fix" but at least it it working.
Hopefully you have resoldered any cold joints or else it may not last.
Benq boards are notorious for bad joints.